Alzheimer's disease is representative of neurodegenerative diseases accompanied by progressive dementia and a loss of cognitive performance, for which no effective therapy has been found to date. Alzheimer's disease is of course one of the most important diseases in the present time of aging society, and development of a therapeutic drug therefor is of paramount importance in medical economics.
On the other hand, abnormal proteins produced due to various forms of external stress, including heat shock and glucose starvation, are known to undergo rapid degradation in vivo via the ubiquitine-proteasome pathway. Ubiquitin is joined to an abnormal protein formed in vivo by a complex enzyme system configured with ubiquitin activation enzyme (E1), ubiquitin binding enzyme (E2), and ubiquitin ligase (E3), and this is followed by repeats of the E1-E2-E3 cycle, whereby a polyubiquitin chain comprising a large number of ubiquitin molecules linked together on branches is formed. This polyubiquitin chain serves as a degradation signal for 26S proteasome, and the abnormal protein is rapidly destroyed.
In recent years, emphasis has been placed on the relationship between various neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, and abnormalities in the ubiquitine-proteasome pathway. For example, in Alzheimer's disease patients, expression of mutant ubiquitin was demonstrated (Science, Vol. 279, pp. 242-247, 1998), and it was reported that proteasome activity was inhibited by this mutant ubiquitin (Proc. Natl. Acad. Sci. USA, Vol. 97, pp. 9902-9906, 2000; J. Cell. Biol, Vol. 157, pp. 417-427, 2002). In autosomal recessive hereditary juvenile Parkinsonism (AR-JP), a hereditary form of Parkinson's disease, Parkin was identified as the etiologic gene (Nature, Vol. 392, pp. 605-608, 1998), and was reported to be a ubiquitin ligase involved in the protein degradation system (Nat. Genet, Vol. 25, pp. 302-305, 2000). Furthermore, as the substrate for Parkin, the Pael (Parkin associated endothelin receptor-like) receptor was identified (Cell, Vol. 105, pp. 891-902, 2001). This receptor is a kind of protein unlikely to form a higher-order structure; if the formation of the higher-order structure of this protein is incomplete, the protein will undergo quick degradation by the action of parkin. However, it has been reported that if this protein degradation system is suppressed due to any abnormality, the Pael receptor with an incompletely formed higher-order structure accumulates in the endoplasmic reticulum, and that cells die of endoplasmic reticulum stress caused by the accumulation.
A feature of the ubiquitin system resides in that ubiquitination occurs very specifically and timely. On the other hand, it is known that a vast number of target proteins are ubiquitinated in cells. Recently, it was found that E3 in the ubiquitine-proteasome pathway exhibits a great molecular diversity, and it was demonstrated that about 1,000 different kinds of E3 are encoded in the human genome. Hence, it was shown that the multiplicity of target proteins is coped with by this number of E3 (Igaku no Ayumi, Vol. 211, pp. 5-11, 2004). E3, among the three groups of enzymes involved in ubiquitination reactions, is a type of enzyme that interacts directly with substrate protein to determine substrate specificity. E3 can be classified into three major groups according to the domain serving as the center of activity: HECT type, RING type, and U-box type. The HECT type and U-box type function as E3 in the form of monomers, whereas the RING type functions in two ways: one functions as a monomer and one functions as a complex with a protein having a RING finger domain, such as Rbx1; the latter is called Cullin-based E3 because it contains a Cullin family protein as the scaffold protein. The SCF complex (S-phase kinase-associated protein 1A (Skp1)-Cullin1-Ring box 1 (Rbx1) complex), which is a kind of this complex, functions as E3 by forming a tetramer of the Skp1, Cullin1, Rbx1, and F-box proteins. Of these proteins, Skp1, Cullin1, and Rbx1 are invariable common components, whereas the F-box protein is a variable component; SCF type ubiquitin ligase is considered to promote the ubiquitination of specific substrate by exchanging F-box proteins, and to date at least 55 kinds of F-box protein have been identified. F-box proteins have an F-box domain for binding to the adapter molecule Skp1 and a domain for binding to substrate protein, and are classified into three types according to the kind of this domain for binding to substrate protein: (i) those having a WD40 repeat domain (Fbw family), (ii) those having a leucine-rich repeat (LRR) (Fbl family), and (iii) others (Fbx family). A kind of F-box protein is considered to bind to some substrate proteins, and to be involved in the ubiquitination of a particular substrate protein. There are some cases in which this binding requires a modification such as phosphorylation. The SCF complex is also diverse in that it is capable of ubiquitinating a very large number of substrate proteins by exchanging F-box proteins; other Cullin-based E3 is known to form a similar complex with the SCF complex, and to be involved in the ubiquitination of other substrate protein as a unique E3 functional molecule.
F-box and leucine rich repeat protein 2 (FBL2) is one of the constituents of the SCF complex, and functions as a F-box protein. FBL2 has been shown to be expressed specifically in the brain and testis (Genomics, Vol. 67, pp. 40-47, 2000), and it has further been reported that the expression of the gene for this protein decreases in the brains of Alzheimer's disease patients (Proc. Natl. Acad. Sci. USA, Vol. 101, pp. 2173-2178, 2004). Although FBL2 is considered to bind to a specific substrate in the leucine-rich repeat region at the C terminus to accentuate ubiquitination, no substrate that binds to FBL2 has been identified to date.
Described in Official Gazette for WO 03/023405 is a therapeutic or prophylactic method for neurodegenerative disease, particularly Alzheimer's disease, in a subject, comprising administering to the subject a therapeutically or prophylactically effective amount of a drug (drugs) that directly or indirectly influences the activity and/or level of (i) the gene that encodes the F-box leucine-rich repeat protein and/or (ii) the transcription product of the gene that encodes the F-box leucine-rich repeat protein and/or (iii) the translation product of the gene that encodes the F-box leucine-rich repeat protein, and/or (iv) a fragment, or derivative, or mutant of (i) to (iii).
Yippee-like 1 (YPEL1) was identified as a gene expressed in the craniofacial genesis stage of mouse viviparity (Genes Cells, Vol. 6, pp. 619-629, 2001), and it has been reported to date that YPEL 1 to 5 exist as genes of the same family (GENE, Vol. 340, pp. 31-43, 2004). Of the genes of the YPEL family, YPEL1 is expressed specifically in the testis and fetal brain, whereas YPEL 2 to 5 are expressed in multiple organs. Because the genes of the YPEL family are localized in the nucleus, particularly abundant in the centrosome, they are considered to be possibly involved in the cell division stage. However, there is only one report that transient expression of YPEL1 in fibroblasts like NIH3T3 cells causes a morphological change like epithelial cells, and no report is available on the function thereof.